1. Statement of the Technical Field
The inventive arrangements relate generally to methods and apparatus for multi-band microstrip antenna operation, and more particularly for dynamically changing the operational band of a microstrip antenna.
2. Description of the Related Art
A wide variety of RF antenna elements are commonly manufactured on dielectric substrate. These include common dipole antenna elements as well as a variety of patch and slot type antennas. The band of frequencies over which such antennas will function is largely determined by the geometry of the antenna element, ground plane spacing and characteristics of the dielectric substrate on which the antenna is formed. In many types of antenna element, antenna equivalent impedance changes significantly with frequency. This results in an impedance mismatch to the feed line when the antenna is operated outside a relatively narrow operational bandwidth. If the impedance of different parts of the circuit do not match, this can result in inefficient power transfer, unnecessary heating of components, and other problems. Consequently, the antenna element may not be usable except over a relatively narrow range of operating frequencies.
Two critical factors affecting the performance of the dielectric substrate material are permittivity (sometimes called the relative permittivity or ∈r) and permeability (sometimes referred to as relative permeability or μr). The relative permittivity and permeability determine the propagation velocity of a signal, which is inversely proportional to √{square root over (μ∈)}. These same factors affect the electrical length of an antenna element. Since antenna elements are typically designed to be a particular geometry and size relative to the wavelength of the operating frequency, the choice of the substrate material affects the overall size of the antenna element.
Moreover, conventional substrate materials typically have a permeability of 1. Accordingly, the choice of relative permittivity value for the dielectric substrate is usually a key design consideration. However, once a dielectric substrate material with a particular permittivity is selected, it is generally a static part of the design and cannot be readily changed. Accordingly, the use of conventional dielectric substrate arrangements have proven to be a limitation in designing antennas.
Further, it is known that the size of an antenna element required for a particular frequency can be reduced by selecting a dielectric substrate with a relatively high permittivity. One method of reducing antenna size is through capacitive loading. This can be accomplished through use of a high permittivity substrate for the array elements. For example, if dipole arms are capacitively loaded by placing them on a substrate of high relative permitivity substrate, the dipole arms can be shortened relative to the arm lengths which would otherwise be needed for a particular frequency using a lower dielectric constant substrate. This effect results because the electrical field in high dielectric substrate portion between the arm portion and the ground plane will be concentrated into a smaller dielectric substrate volume.
However, one drawback of this approach is that the radiation efficiency is often reduced. The radiation efficiency is the frequency dependent ratio of the power radiated by the antenna to the total power supplied to the antenna. In the case of a dipole, for example, a shorter arm length reduces the radiation resistance, which is approximately equal to the square of the arm length for a “short” (less than ½ wavelength) dipole antenna as shown below:Rr=20 π2(l/λ)2where l is the electrical length of the antenna line and λ is the wavelength of interest.
A conductive trace comprising a single short dipole can be modeled as an open transmission line having series connected radiation resistance, an inductor, a capacitor and a resistive ground loss. The radiation efficiency of a dipole antenna system, assuming a single mode can be approximated by the following equation:   E  =            R      r              (                        R          r                +                  X          L                +                  X          C                +                  R          L                    )      Where
E is the efficiency
Rr is the radiation resistance
XL is the inductive reactance
XC is the capacitive reactance
XL is the ohmic feed point ground losses and skin effect
The radiation resistance is a fictitious resistance that accounts for energy radiated by the antenna. The inductive reactance represents the inductance of the conductive dipole lines, while the capacitor is the capacitance between the conductors. The other series connected components simply turn RF energy into heat, which reduces the radiation efficiency of the dipole.
From the foregoing, it can be seen that the constraints of a dielectric substrate having selected relative dielectric properties often results in design compromises that can negatively affect the electrical performance and/or physical characteristics of the overall circuit. An inherent problem with the conventional approach is that, at least with respect to the substrate, the only control variable for line impedance is the relative permittivity. This limitation highlights another important problem with conventional substrate materials, i.e. they fail to take advantage of the other factor that determines characteristic impedance, namely Ll, the inductance per unit length of the transmission line.